Overexpression of prolylcarboxypeptidase enhances plasma prekallikrein activation on Chinese hamster ovary cells

Plasma prekallikrein (PK) complexes with its receptor, high-molecular-weight kininogen (HK), on human umbilical vein endothelial cells (HUVEC). When assembled on endothelial cells, PK is activated to plasma kallikrein independent of factor XIIa by the serine protease prolylcarboxypeptidase (PRCP, Km= 9 nM). PRCP was shown to be a PK activator when isolated from HUVEC (J Biol Chem 277: 17962-17969, 2002) and produced as a recombinant protein (Blood 103: 4554-4561, 2004). To additionally confirm that human PRCP is a physiological PK activator, PRCP was overexpressed in Chinese hamster ovary (CHO) cells. CHO cells were transfected with full-length PRCP under the control of a cytomegalovirus promoter, and CHO recombinant PRCP was expressed as a fusion protein with COOH-terminal enhanced green fluorescence protein (EGFP). The presence of recombinant PRCP in transfected CHO cells was detected by real-time RT-PCR, immunoblot, and immunoprecipitation. PRCP mRNA and PK activation were two- to threefold higher in transfected than in control CHO cells. The increase in PRCP-induced PK activation in the transfected CHO cells paralleled the increase in PRCP antigen expression, as determined by anti-PRCP and anti-green fluorescence protein antibodies. PK activation of the transfected cells was blocked by small interfering RNA to PRCP. Anti-PRCP antibody and Z-Pro-Pro-aldehyde dimethyl acetate also blocked PK activation (IC50= 0.01 and 7.0 mM, respectively). Localization of PRCP in intact cells observed via confocal microscopy and flow cytometry also confirmed overexpression of PRCP on the external membrane. These investigations independently confirm that PRCP is expressed on cell membranes and that PRCP expression increases PK activation.     

Characterization of a bifunctional peptidylglycine alpha-amidating enzyme expressed in Chinese hamster ovary cells.

Peptidylglycine alpha-amidating enzyme (alpha-AE) catalyzes the conversion of glycine-extended prohormones to their biologically active alpha-amidated forms. We have derived a clonal Chinese hamster ovary cell line that secretes significant quantities of active alpha-AE. Enzyme production was increased by selection for methotrexate-resistant cells expressing a dicistronic message. Amplification of the alpha-AE gene was monitored by Southern blot analysis, enzyme activity, and immunoreactive protein throughout the selection process. The soluble enzyme is bifunctional as determined by the ability to convert either the glycine-extended substrate, dansyl-Tyr--Val--Gly, or the intermediate, dansyl-Tyr--Val--alpha-hydroxyglycine, to the dansyl-Tyr--Val--NH2 product. The recombinant alpha-AE was purified by a simple two-step chromatographic process. The purified enzyme is partially glycosylated and the glycosylated and nonglycosylated forms of the enzyme were separated on a Con A-Sepharose column. The kinetic constants for dansyl-Tyr--Val--Gly, dansyl-Tyr--Val--alpha-hydroxyglycine, ascorbate, and catechol were the same for both forms of alpha-AE. In addition, mimosine is competitive vs ascorbate with K(is) = 3.5 microM for the nonglycosylated alpha-AE and K(is) = 4.2 microM for the glycosylated alpha-AE. Therefore, the presence or absence of asparagine-linked oligosaccharide does not affect the catalytic efficiency of the enzyme. Overexpression of the recombinant enzyme in CHO cells greatly enhances expression of the endogenous gene, implicating a feedback mechanism on the alpha-AE gene.     

Dolichol metabolism in Chinese hamster ovary cells.

The addition of oligosaccharide to asparagine residues of soluble and membrane-associated proteins in eukaryotic cells involves a polyisoprenoid lipid carrier, dolichol. In Chinese hamster ovary cells, the major isomer of this polyisoprenol has 19 isoprenyl units, the terminal one being saturated. Our laboratory has developed a procedure to analyze the levels and nature of the cell's dolichyl derivatives. Chinese hamster ovary cells contain predominately activated, anionic dolichol derivatives, such as oligosaccharyl pyrophosphoryldolichol, monoglycosylated phosphoryldolichols, and dolichyl phosphate. Our studies show that in growing cells there is continual synthesis of total dolichol. Also, preliminary data suggest there is no catabolism or secretion of this lipid. The level of dolichyl phosphate did not change significantly under a variety of conditions where the levels of enzyme activities utilizing dolichyl phosphate did change. These results suggested that these enzymes had access to the same pool of dolichyl phosphate and had similar Km values for this lipid.     

Overexpression of recombinant human antithrombin III in Chinese hamster ovary cells results in malformation and decreased secretion of recombinant protein

Overexpression of recombinant proteins in animal cells is commonly achieved by using gene amplification techniques. Gene amplified cells possess up to several thousand genes coding for the target protein. Constitutive expression of these genes leads to high levels of the corresponding mRNA species and the immature protein in the cell. Inefficient processing of these precursors may result from their great abundance in the cell. To study the influence of elevated intracellular levels of a recombinant protein on its maturation and secretion, we examined the maturation and secretion of human antithrombin III (hATIII) in Chinese hamster ovary (CHO) cells at different levels of gene amplification. No loss of vitality was caused by elevated secretion of hATIII. As the intracellular hATIII content increased, the efficiency of hATIII secretion decreased steadily. The state of intracellular hATIII from the different cell lines was studied by determining the specific heparin cofactor activity of hATIII. Intracellular hATIII from the highest amplified cell line displayed a lowered specific heparin cofactor activity indicating the presence of malfolded, only partially folded, or incompletely or incorrectly posttranslationally modified hATIII in this cell line. Thus, the ability of CHO cells to fold and/or introduce posttranslational modifications and subsequently to secrete the recombinant protein becomes saturated, and therefore these processes may become limiting for protein secretion at highly elevated expression levels. This limitation was not due to a general exhaustion of the secretory capacity of the cells because hATIII constituted only a minor fraction of the secreted proteins, even at high expression levels. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 53: 547-559, 1997.     

Internalization of ricin in Chinese hamster ovary cells.

Internalization of ricin into Chinese hamster ovary cells has been investigated. Combined treatment with galactose and pronase at 0 degrees C resulted in a complete release of surface-bound [125I]ricin into the media. Galactose-pronase-resistant cell-bound [125I]ricin represents internalized ricin molecules inside the cells. The internalization process is time, temperature, and concentration dependent. The pH optimum of internalization of ricin is about pH 7. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis has revealed that intact ricin molecules are internalized. Neither reduction nor proteolytic processing of ricin is required for the entry of ricin into Chinese hamster ovary cells.     

Two Chinese hamster ovary glycosylation mutants affected in the conversion of GDP-mannose to GDP-fucose

A biochemical basis for the pea and lentil lectin resistance of two Chinese hamster ovary (CHO) cell mutants, Lec13 and Lec13A, was investigated. Studies of the G glycopeptides of vesicular stomatitis virus grown in the mutants indicated that Lec13 cells essentially lack the ability to add fucose to complex carbohydrates while Lec13A cells synthesize significant proportions of fucosylated, complex moieties. However, both mutants were known to be reverted to lectin sensitivity by growth in L-fucose, making them similar to the mouse lymphoma mutant, PLR1.3, which is defective in the conversion of GDP-mannose to GPD-fucose [M. L. Reitman, I. S. Trowbridge, and S. Kornfeld (1980) J. Biol. Chem. 255, 9900-9906]. Optimal conditions for the production of GDP-fucose from GDP-mannose by CHO cytosol were found to occur at pH 8 in the presence of 7.5 microM GDP-mannose, 15 mM Mg2+, 0.2 mM NAD+, 0.2 mM NADPH, 10 mM niacinamide, 5 mM ATP, and 50 mM Tris-HCl. Under these conditions, Lec13 cytosol produced no detectable GDP-fucose nor GDP-sugar intermediates while Lec13A cytosol produced significant quantities of both. Mixing experiments with Lec13 cytosol identified the first enzyme of the conversion pathway (GDP-mannose 4,6-dehydratase, EC 4.2.1.47) as the site of the block. In addition to being markedly reduced, the Lec13A 4,6-dehydratase activity was relatively insensitive to changes in pH in comparison to the activity in parental cytosol, suggesting that Lec13A cells might possess a structurally altered GDP-mannose 4,6-dehydratase enzyme.     

Purification and characterization of recombinant human testis angiotensin-converting enzyme expressed in Chinese hamster ovary cells.

Enzymatically active human testis angiotensin-converting enzyme (ACE) was expressed in Chinese hamster ovary (CHO) cells stably transfected with each of three vectors: p omega-ACE contains a full-length testis ACE cDNA under the control of a retroviral promoter; and pLEN-ACEVII and pLEN-ACE6/5, in which full-length and membrane anchor-minus testis ACE cDNAs, respectively, are under the control of the human metallothionein IIA promoter and SV40 enhancer. In every case, active recombinant human testis ACE (hTACE) was secreted in a soluble form into the culture media, up to 2.4 mg/liter in the media of metal-induced, high-producing clones transfected with one of the pLEN vectors. In addition, membrane-bound recombinant enzyme was recovered from detergent extracts of cell pellets of CHO cells transfected with either p omega-ACE or pLEN-ACE-VII. Recombinant converting enzyme was purified to homogeneity by single-step affinity chromatography of conditioned media and detergent-extracted cell pellets in 85 and 70% overall yield, respectively. Purified hTACE from all sources comigrated with the native testis isozyme on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with M(r) approximately 100 kDa. The native and recombinant proteins cross-reacted equally with anti-human kidney ACE antiserum on Western blotting. The catalytic activity of recombinant angiotensin-converting enzyme, in terms of angiotensin I and 2-furanacryloyl-Phe-Gly-Gly hydrolysis, chloride activation, and lisinopril inhibition, was essentially identical to that of the native enzyme. The facile recovery in high yield of fully active hTACE from the media of stably transfected CHO cells provides a suitable system for investigating structure-function relationships in this enzyme.     

Platinum-induced mutations to 8-azaguanine resistance in Chinese hamster ovary cells.

6 platinum (Pt) compounds were compared in suspension cultured Chinese hamster ovary (CHO-S) cells with respect to their inhibition of growth, their reduction of cloning efficiency, and their induction of mutants resistant to 200 microM (30 micrograms/ml) 8-azaguanine (8-AG) and 3 mM ouabain (OUA), respectively. The toxicity of these compounds can be ranked by the medium concentrations which decrease suspension growth/or cloning efficiency by 50%: cis-Pt(NH3)2-Cl2 (0.9/1.5 microM) greater than Pt(SO4)2 + methylcobalamin (MeB-12) methylation product (20/10 microM) greater than K2PtCl4 (32/50 microM) = K2PtCl6 (34/50 microM) = MePtCl2-3 (60/50 microM) greater than Pt(SO4)2 (66/105 microM). Following 20 h exposures to concentrations which resulted in relative survivals of 80-2%, none of the foregoing compounds increased consistently the frequency of OUA(R) mutants above the spontaneous frequency (6.0 x 10(-6)). Parallel treatments with 800 microM (100 micrograms/ml) ethyl methanesulfonate (EMS) increased the OUA(R) mutant frequency 10--12-fold. Using 8-AG for mutant selection, dose-dependent increases of 5--7-fold above the spontaneous frequency (3--8 x 10(-5) were obtained with cis-Pt(NH3)2Cl2, Pt(S04)2, and the product from Pt(SO4)2 + MeB-12. Identical 20 h exposures to varying amounts of K2PtCl4, K2PtCl6, and MePtCl2-3 did not induce 8-AG(R) mutants. Optimal detection of Pt-induced 8-AG(R) mutants required 7 post-treatments, expression doublings in suspension culture. Under our selection conditions 8/8 spontaneous and 24/24 Pt-induced 8-AG(R) variants contained reduced hypoxanthine-guanine phosphoribosyl transferase (HGPRT) specific activities (means ranging from 3 to 11% of the parental CHO-S cells). When compared from linear plots of the 8-Ag(r) frequency against the initial medium concentration, cis-Pt(NH3)2Cl2 is 134 times and Pt(SO4)2 si 3.5 times more mutagenic than EMS. However, on a cell-survival basis EMS is 8--10-fold more mutagenic than these two Pt-compounds. 6-Thioguanine (10 microM) can be substituted for 8-AG to assay mutant induction by cis-Pt(NH3)2Cl2 and Pt(SO4)2 in CHO-S cells. The sensitivity of the CHO-S HGPRT locus for detecting mutagenesis by Pt complexes can be increased several fold by continuous subculture in the presence of these agents for 10--25 population doublings. By this procedure K2PtCl6 is seen to be weakly mutagenic and 20 microM Pt(SO4)2 produces 8-AG(R) mutants at frequencies requiring 7--8-fold higher concentrations when a fixed 20 h exposure is used.     

Hydroxyeicosatetraenoic acid oxidation in Chinese hamster ovary cells: a peroxisomal metabolic pathway

To evaluate the peroxisomal requirement for beta-oxidation of hydroxyeicosatetraenoic acids (HETES), we tested 5-, 12- and 15-HETE oxidation in wild-type and mutant Chinese hamster ovary (CHO) cells. Mutant CHO cells contain peroxisomal ghosts, have random cytosolic localization of catalase and lack two of the enzymes necessary for peroxisomal beta-oxidation. Reverse-phase HPLC indicated that 33% of 12-HETE radioactivity was converted by wild-type CHO cells during a 2 h incubation to one major and several minor polar metabolites. Wild-type CHO cells also converted 15-HETE to one major and several minor polar metabolites. Neither 12- nor 15-HETE were converted to any metabolites by the mutant CHO cell lines, despite appreciable cellular uptake of these hydroxyeicosanoids. 5-HETE was not converted to any metabolic products by either the wild-type or the mutant CHO cells. Docosahexaenoic acid beta-oxidation was substantially reduced in the mutants as compared to the wild-type cells, palmitic acid beta-oxidation was reduced to an intermediate extent in the mutants, but octanoate beta-oxidation and citrate synthase activity were not impaired. Protein immunoblotting for mitochondrial manganese superoxide dismutase indicated a single band of identity at 20 kDa in both wild-type and mutant CHO cells. Since mutant CHO cells fail to convert 12- and 15-HETE to oxidative metabolites but contain normal mitochondrial enzymatic activities, intact peroxisomes appear to be the organelle responsible for HETE oxidation.     

Tetracycline-regulated overexpression of glycosyltransferases in Chinese hamster ovary cells.

The glycosylation patterns of recombinant therapeutic glycoproteins can be engineered by overexpression of glycosyltransferases in the host cells used for glycoprotein production. Most prior glycosylation engineering experiments have involved constitutive expression of cloned glycosyltransferases. Here we use tetracycline-regulated expression of two glycosyltransferases, N-acetylglucosaminlytransferases III and V (GnTIII and GnTV) to manipulate glycoform biosynthesis in Chinese hamster ovary (CHO) cells and to study the effect of glycosyltransferase overexpression on this host. The amount of GnTIII and GnTV in these cells, and the glycosylation patterns of several cellular glycoproteins, could be controlled simply by manipulating the concentration of tetracycline in the culture medium. Using this system, it was found that overexpression of either GnTIII or GnTV to high levels led to growth inhibition and was toxic to the cells, indicating that this may be a general feature of glycosyltransferase overexpression. This phenomenon has not been reported previously, probably due to the widespread use of constitutive promoters, and should be taken into account when designing vectors for glycosylation engineering. The growth inhibition effect sets an upper limit to the level of glycosyltransferase overexpression, and may thereby also limit the maximum extent of in vivo modification of poorly accessible glycosylation sites. Also, such inhibition implies a bound on constitutive glycosyltransferase expression which can be cloned.     

Squalene synthase-deficient mutant of Chinese hamster ovary cells.

Squalene synthase (farnesyldiphosphate:farnesyldiphosphate farnesyltransferase, EC 2.5.1.21) converts farnesyl pyrophosphate to squalene, the first metabolic step committed solely to the biosynthesis of sterols. Using a fluorescence-activated cell sorting technique designed to screen for cells defective in the regulated degradation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, we isolated a squalene synthase-deficient mutant of Chinese hamster ovary cells. The mutant cell line, designated SSD, exhibits less than 7% of the squalene synthase activity of the parental cell line, CHO-HMGal. Both the SSD and the parental cells stably express HMGal, a model protein for studying the regulated degradation of HMG-CoA reductase, which consists of the membrane domain of HMG-CoA reductase fused to bacterial beta-galactosidase (Skalnik, D. G., Narita, H., Kent, C., and Simoni, R. D. (1988) J. Biol. Chem. 263, 6836-6841). In this study, the regulatory effects of mevalonate and compactin on the activity levels of HMGal are substantially reduced in SSD cells as compared to the parental cell line. In lipid-poor medium, SSD cell growth is arrested. The rate of [3H]acetate incorporation into cholesterol for the mutant SSD cells is less than 2% of the rate for the parental cells. However, the incorporation of [3H] squalene into sterols is essentially wild type for SSD cells. When the mutant SSD cells are fed [3H]acetate, radioactivity accumulates in farnesol, much of which is secreted into the medium. By growing SSD cells in lipid-poor medium, a revertant cell type, designated SSR, was isolated. In every assay performed the revertant SSR cells exhibited a phenotype that was essentially wild type, demonstrating that the SSD mutant phenotype was the result of a single mutation.     

The selection of virus-resistant Chinese hamster ovary cells.

We have selected Chinese hamster ovary (CHO) cells resistant to infection by encephalomycarditis (EMC) virus. Thus far, we have obtained five lines resistant to EMC, all of which manifest different phenotypes. Three of the five are not persistently infected with virus, while two lines produce infectious virus and grow in its presence. The nonpersistently infected lines exhibit different resistance profiles to the other viruses we have tested, and they are stable in nonselective growth conditions. Their resistance appears to be due to a genetic alteration in the cell.     

Pesticide clastogenicity in Chinese hamster ovary cells

Paraquat, alachlor, butachlor, phorate and monocrotophos, several of the most extensively used pesticides in Taiwan, were investigated for their clastogenicity using chromosome aberration (CAb) induction in Chinese hamster ovary (CHO) cells. Significance levels of the binomial trend analysis and binomial mutagenicity data test were two criteria for the summary judgement of the pesticide clastogenicity. Except for phorate, all pesticides tested were clastogenic to CHO cells in the absence of in vitro metabolic activation by S9. 5 microliters/ml rat-liver extract, S9, were used as the source of in vitro metabolic activation. 3 different outcomes were found after the addition of S9. Paraquat: significant decrease in induced CAbs. Monocrotophos: concomitant occurrence of decreased cytotoxicity and increased clastogenicity. Alachlor, butachlor and phorate: increased cytotoxicities with no sign of enhancement in clastogenicity.     

Chinese hamster ovary cells replicate adenovirus deoxyribonucleic acid.

Chinese hamster ovary (CHO) cells infected with adenovirus type 2 (Ad2) produced amounts of viral deoxyribonucleic acid (DNA) equal to that synthesized in permissively infected HeLa cells. However, there was 6,000-fold less virion produced in CHO cells. Since the structural viral polypeptides were not detected by pulse-labeling CHO cells at various times postinfection, the block in virion formation is located between the synthesis of viral DNA and late proteins. Extracts of CHO cells could also function in a recently reported in vitro Ad2 DNA synthesis system which is dependent upon the addition of exogenous Ad2 DNA covalently linked to a 5'-terminal protein (Ikeda et al., Proc. Natl. Acad. Sci. U.S.A. 77:5827-5831, 1980). Extracts of infected CHO cytoplasm were able to complement uninfected CHO nuclear extracts to synthesize viral DNA on Ad2 templates. This in vitro replication system has the potential to probe host DNA synthesis requirements as well as viral factors.     

Mevalonate deprivation leads to aneuploidy in Chinese hamster ovary cells

After exposure to compactin, the competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, 22% of CHO-K1 cells contained abnormally high numbers of chromosomes. In two populations of cells selected for compactin resistance 31 and 33% of the cells contain more than 22 chromosomes. Some cell lines isolated from these populations have the wild type chromosome number of 20-21, while others have a broad distribution of chromosome number, often with a mean around 36-40. Finally, Chinese hamster ovary cells that are mutant for 3-hydroxy-3-methylglutaryl-CoA reductase and therefore auxotrophic for mevalonate were starved for that compound. This treatment also increased the number of cells containing extra chromosomes. These results indicate that interruption of the cellular supply of mevalonate results in abnormal chromosome number.     

Evidence for allelic exclusion in Chinese hamster ovary cells.

Earlier results suggested that the functional hemizygosity of genes in pseudodiploid Chinese hamster ovary (CHO) cells is due to the silencing of one allele by DNA methylation. From this one could make a strong prediction that we have now been able to confirm by genetic experiments, using thymidine kinase (TK) alleles. TK- mutants induced by ethylmethane sulphonate (EMS) were all revertible to TK+ at high frequency by the demethylating agent 5-azacytidine (5-aza-CR). This revertibility was due to reactivation of a silent nonmutant TK allele. Further mutagenesis by EMS yielded TK- derivatives that were no longer revertible by 5-aza-CR; these are assumed to have mutations in both alleles. TK- cells were also transfected with equine herpes virus TK+ DNA, and the TK+ derivatives were shown to be markedly less stable than cells with the normal TK+ gene. CHO cells lack metallothionein activity (sensitive to cadmium), and also require proline for growth, because genes have become silenced during the establishment of the cell line. In both cases 5-aza-CR reactivates these genes to give the cadmium resistant and proline independent phenotypes. Long-term experiments with reactivants in the absence of selection showed that the genes become silent, presumably as a result of de novo methylation. A strain resistant to cytosine arabinoside (araCR) was also resistant to 5-azadeoxycytidine (5-aza-CdR), but not to 5-aza-CR, which would be expected if the araCR strain lacked deoxycytidine kinase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure of the dihydrofolate reductase gene in Chinese hamster ovary cells.

Overlapping recombinant lambda 1059 phages carrying regions of the dhfr locus from the amplified Chinese hamster ovary (CHO) cell clone MK42 have been isolated. In addition, dhfr cDNAs from this cell line have been cloned into plasmid pBR322. Restriction analysis of these recombinant molecules has led to a map of the Chinese hamster dhfr gene. This gene has a minimum size of 26 kb and contains six exons as defined by hybridization to a combination of mouse and CHO cDNA probes. The latter probes reveal 3' exonic sequences that are not present in mouse cDNA. The CHO dhfr gene thus extends about 700 bp further 3' than in the mouse, consistent with the larger size of the hamster mRNA. At least five intervening sequences are present, of approximate sizes: 0.3, 2.5, 8.6, 2.6 and 9.4 kb. Four sequences from highly repeated families are situated in introns within the dhfr gene. The overall structure of this gene is strikingly similar to that of the mouse. Evolutionary conservation of interrupted gene structure among mammals thus extends to genes that code for household enzymes as well as specialized or structural proteins.     

Characterization of the diphtheria toxin-resistance system in Chinese hamster ovary cells.

Variations in two general classes of diphtheria toxin-resistant mutants which may be selected from Chinese hamster ovary (CH0-K1) cells and the conditions for their selection are described. The resistance of class I mutants can be overcome with increasing concentrations of toxin. Their entire complement of EF-2 is susceptible to ADP-ribosylation by toxin. Class I includes those strains in which resistance resides at the level of the plasma membrane. The resistance of class II, translational, mutants cannot be overcome by high concentrations of toxin, as all, or a portion, of their EF-2 is insensitive to the action of diphtheria toxin and Pseudomonas exotoxin A. Adjustment of the concentration of toxin used to select resistant mutants can be used to regulate the class of mutant recovered. Metabolic cooperation between cells does not affect recovery of either class I or class II mutants. Resistance is stable in class I strains, but class IIb strains, which possess 50% resistant and 50% sensitive EF-2, display a transient high level of resistance which is retained for varying lengths of time following exposure to toxin. Class IIa strains, which possess 100% resistant EF-2, grow normally in saturating concentrations of toxin, but class IIb strains grow at a reduced rate. Evidence is presented which suggests that the gene for EF-2 is functionally diploid in CHO-K1 cells.